Low vinyl SSBR (Solution Styrene Butadiene) is difficult to produce in a continuous polymerization due to the kinetics of copolymerization. Fast kinetics is required in a continuous polymerization to get an acceptable production rate and high conversion (low residual monomer concentration). Low vinyl SSBR is required to make a product with a high level of skid resistance and high abrasion resistance. It is not possible to produce low vinyl SSBR without special polar agents, so called randomizers, such as TMEDA (N,N,N′,N′-tetramethylethylene diamine). However, the use of TMEDA in a continuous polymerization process, results in a SSBR product which contains a very high block styrene content. Such polymers are not suitable for tire application, because these polymers have a decreased rolling resistance.
Ditetrahydrofurylpropane (DTHFP) has been used as a randomizer for SSBR polymerization. However, when using DTHFP in batch processes for the production of low vinyl SSBR, the monomer conversion has to be kept below 95% to get SSBR with a low block styrene content (for example, see EP0215256). If the final conversion is low, a high amount of unreacted monomers remains in the final polymer solution, and an additional solvent purification process is required. There is a need for a continuous polymerization process to produce low vinyl SSBR with styrene blocks (for example, a styrene block content below 8 weight percent), and which achieves a high monomer conversion with minimal levels of residual monomers.
U.S. Publication No. 2009/0111933 discloses an oil-extended polymer composition comprising 100 parts by weight of a butadiene-based polymer having the following: a) a number average molecular weight (Mn) from 500,000 to 700,000, b) a molecular weight distribution (Mw/Mn) from 1.8 to 2.5, c) a monomodal distribution of polymer components having a molecular weight of 100,000 or more, and d) a vinyl content of the butadiene unit of 20 to 80 mole percent. The composition also contains a functional group-containing polymer component than can be adsorbed on silica, and an extender oil. The polymers described in this patent have however a high vinyl content which results in a final product with a low abrasion resistance. The use of a functionalization agent is claimed in order to optimize the polymer properties.
European Patent Application No. EP0215256 A1 discloses a process for preparing randomized copolymers of conjugated dienes and vinyl aromatic hydrocarbons, by using oligomeric oxolanyl alkanes, in amounts which effectively promote the randomization of the copolymers so produced. As previously described, in this patent, a low vinyl SSBR is obtained only in combination with a low conversion. A low conversion requires an additional solvent purification process to remove unreacted monomers.
U.S. Publication No. 2007/0219316 discloses a continuous polymerization process, carried out in the presence of at least two reactors in series, for the preparation of statistical vinyl arene/conjugated diene copolymers. The vinyl arene content range from 15 to 50 percent by weight. The polymerization is a copolymerization of vinyl arene/conjugated diene monomers, under isothermal conditions, in a hydrocarbon solvent, at a temperature ranging from 30° C. to 120° C., in the presence of at least one initiator and a 2-methoxy ethyl tetra-hydrofuran (THFA-ethyl) modifier.
U.S. Pat. No. 4,672,097 discloses a process for preparing randomized copolymers of conjugated dienes and vinyl aromatic hydrocarbons, and which have low vinyl contents, by using an anionic initiator system, comprising alkali metal trihydrocarbyl magnesiates, optionally together with an organolithium compound. However such initiating system are not very soluble in hydrocarbon solvents and thus, not suitable in continuous polymerizations. In particular, in order to obtain a low vinyl level combined with a good randomization and high conversion, a high temperature has to be used, which causes the metallation and/or deactivation of the living chain ends.
Additional polymerizations and rubbers are disclosed in the following: U.S. Pat. Nos. 3,554,997, 4,429,091, 5,679,751, and 6,372,863, and European Patent Application No. 0019100A1.
As discussed, there remains a need for continuous polymerization processes to produce a low vinyl SSBRs with low block styrene contents. There is a further need for such a polymerizations that achieve high monomer conversions with minimal levels (for example, below 1000 ppm) of residual monomers. These needs have been met by the following invention.